Foursquare has hit 3 million users for its location-based social network. Facebook  launched a rival location service, Facebook Places, in mid-August. But that apparently hasn’t lessened interest in Foursquare’s service, as some had feared. Foursquare’s growth is still accelerating, the numbers show.

As Techcrunch noted, Foursquare took a year to reach one million users, three months to hit its second million, and six weeks to hit its third million. Of course, Facebook Places is only 12 days old, and you can’t expect the larger social network to take down Foursquare overnight. But it will be interesting to see if there is a win-win situation here, where both services can grow, or if one will grow only at the expense of the other.

Facebook says it designed its Places applications programming interface as a platform for services like Foursquare, Gowalla and Booyah. Foursquare raised $20 million at a $95 million pre-money valuation from Andreessen Horowitz, Union Square Ventures, and O’Reilly AlphaTech Ventures. Rival service Booyah’s MyTown hit 3 million users earlier this month, and Loopt passed 4 million users in July.

[picture credit: Techcrunch]